Active suspension

Type of automotive suspension

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An active suspension is a type of automotive suspension that uses an onboard control system to control the vertical movement of the vehicle's wheels and axles relative to the chassis or vehicle frame, rather than the conventional passive suspension that relies solely on large springs to maintain static support and dampen the vertical wheel movements caused by the road surface. Active suspensions are divided into two classes: true active suspensions, and adaptive or semi-active suspensions. While adaptive suspensions only vary shock absorber firmness to match changing road or dynamic conditions, active suspensions use some type of actuator to raise and lower the chassis independently at each wheel.

These technologies allow car manufacturers to achieve a greater degree of ride quality and car handling by keeping the chassis parallel to the road when turning corners, preventing unwanted contacts between the vehicle frame and the ground (especially when going over a depression), and allowing overall better traction and steering control. An onboard computer detects body movement from sensors throughout the vehicle and, using that data, controls the action of the active and semi-active suspensions. The system virtually eliminates body roll and pitch variation in many driving situations including cornering, accelerating and braking. When used on commercial vehicles such as buses, active suspension can also be used to temporarily lower the vehicle's floor, thus making it easier for passengers to board and exit the vehicle.

Principle

Skyhook theory is that the ideal suspension would let the vehicle maintain a stable posture, unaffected by weight transfer or road surface irregularities, as if suspended from an imaginary hook in the sky continuing at a constant altitude above sea level, therefore remaining stable.

Since an actual skyhook is obviously impractical,[1] real active suspension systems are based on actuator operations. The imaginary line (of zero vertical acceleration) is calculated based on the value provided by an acceleration sensor installed on the body of the vehicle (see Figure 3). The dynamic elements comprise only the linear spring and the linear damper; therefore, no complicated calculations are necessary.[2][3]

A vehicle contacts the ground through the spring and damper in a normal spring damper suspension, as in Figure 1. To achieve the same level of stability as the Skyhook theory, the vehicle must contact the ground through the spring, and the imaginary line with the damper, as in Figure 2. Theoretically, in a case where the damping coefficient reaches an infinite value, the vehicle will be in a state where it is completely fixed to the imaginary line, thus the vehicle will not shake.

Active

Active suspensions, the first to be introduced, use separate actuators which can exert an independent force on the suspension to improve the riding characteristics. The drawbacks of this design are high cost, added complication and mass of the apparatus, and the need for frequent maintenance on some implementations. Maintenance can require specialised tools, and some problems can be difficult to diagnose.

Hydraulic actuation

Hydraulically actuated suspensions are controlled with the use of hydraulics. The first example appeared in , with the hydropneumatic suspension developed by Paul Magès at Citroën. The hydraulic pressure is supplied by a high pressure radial piston hydraulic pump. Sensors continually monitor body movement and vehicle ride level, constantly supplying the hydraulic height correctors with new data. In a matter of a few milliseconds, the suspension generates counter forces to raise or lower the body. During driving maneuvers, the encased nitrogen compresses instantly, offering six times the compressibility of the steel springs used by vehicles up to this time.[4]

In practice, the system has always incorporated the desirable self-levelling suspension and height adjustable suspension features, with the latter now tied to vehicle speed for improved aerodynamic performance, as the vehicle lowers itself at high speed.

This system performed remarkably well in straight ahead driving, including over uneven surfaces, but had little control over roll stiffness.[5]

Millions of production vehicles have been built with variations on this system.

Electronic actuation of hydraulic suspension

Colin Chapman developed the original concept of computer management of hydraulic suspension in the s to improve cornering in racing cars. Lotus fitted and developed a prototype system to a Excel with electro-hydraulic active suspension, but never offered it for sale to the public, although many demonstration cars were built for other manufacturers.

Sensors continually monitor body movement and vehicle ride level, constantly supplying the computer with new data. As the computer receives and processes data, it operates the hydraulic servos, mounted beside each wheel. Almost instantly, the servo-regulated suspension generates counter forces to body lean, dive, and squat during driving maneuvers.

In , Nissan installed a hydraulic supported MacPherson strut based setup, called Full-Active Suspension that was used in the Nissan Q45 and President. The system used a hydraulic oil pump, a hydraulic cylinder, an accumulator and damping valve, which connected two independent circuits for the front and rear strut assemblies. The system would then recover motion energy to balance the car continuously.[6] The system was revised and is now called Hydraulic Body Motion Control System, installed on the Nissan Patrol and Infiniti QX80.

Williams Grand Prix Engineering prepared an active suspension, devised by designer-aerodynamicist Frank Dernie, for the team's Formula 1 cars in , creating such successful cars that the Fédération Internationale de l'Automobile decided to ban the technology to decrease the gap between Williams F1 team and its competitors.[7]

Computer Active Technology Suspension (CATS) co-ordinates the best possible balance between ride quality and handling by analysing road conditions and making up to 3,000 adjustments every second to the suspension settings via electronically controlled dampers.

The Mercedes-Benz CL-Class (C215) introduced Active Body Control, where high pressure hydraulic servos are controlled by electronic computing, and this feature is still available. Vehicles can be designed to actively lean into curves to improve occupant comfort.[8][9]

Active anti-roll bar

Active anti-roll bar stiffens under command of the driver or suspension electronic control unit (ECU) during hard cornering. First production car was Mitsubishi Mirage Cyborg in .

Electromagnetic recuperative

In fully active electronically controlled production cars, the application of electric servos and motors married to electronic computing allows for flat cornering and instant reactions to road conditions.

The Bose Corporation has a proof of concept model. The founder of Bose, Amar Bose, had been working on exotic suspensions for many years while he was an MIT professor.[10]

Electromagnetic active suspension uses linear electromagnetic motors attached to each wheel. It provides extremely fast response, and allows regeneration of power consumed, by using the motors as generators. This nearly surmounts the issues of slow response times and high power consumption of hydraulic systems. Electronically controlled active suspension system (ECASS) technology was patented by the University of Texas Center for Electromechanics in the s[11] and has been developed by L-3 Electronic Systems for use on military vehicles.[12] The ECASS-equipped Humvee exceeded the performance specifications for all performance evaluations in terms of absorbed power to the vehicle operator, stability and handling.

Active Wheel

Adaptive and semi-active

Adaptive or semi-active systems can only change the viscous damping coefficient of the shock absorber, and do not add energy to the suspension system. While adaptive suspensions have generally a slow time response and a limited number of damping coefficient values, semi-active suspensions have time response close to a few milliseconds and can provide a wide range of damping values. Therefore, adaptive suspensions usually only propose different riding modes (comfort, normal, sport...) corresponding to different damping coefficients, while semi-active suspensions modify the damping in real time, depending on the road conditions and the dynamics of the car. Though limited in their intervention (for example, the control force can never have different direction than the current vector of velocity of the suspension), semi-active suspensions are less expensive to design and consume far less energy. In recent times, research in semi-active suspensions has continued to advance with respect to their capabilities, narrowing the gap between semi-active and fully active suspension systems.

Solenoid/valve actuated

This type is the most economic and basic type of semi-active suspensions. They consist of a solenoid valve which alters the flow of the hydraulic medium inside the shock absorber, therefore changing the damping characteristics of the suspension setup. The solenoids are wired to the controlling computer, which sends them commands depending on the control algorithm (usually the so-called "Sky-Hook" technique).[citation needed]

This type of system is used in Cadillac's Computer Command Ride (CCR) suspension system. The first production car[22] was the Toyota Soarer with semi-active Toyota Electronic Modulated Suspension, from .

In , Nissan introduced a shock absorber using a similar version, called "Super Sonic Suspension," adding an ultrasonic sensor that would provide information that a microcomputer would then interpret, combined with information from the steering, brakes, throttle, and vehicle speed sensor. The adjustment information signals would then modify the shock absorbers when a driver-controlled switch was placed in "Auto". The automatic adjustment could be limited if the switch was placed in "Soft," "Medium," or "Hard" settings. A modified version that didn't use the sonar module was also used, allowing the settings to be manually selected.[23][24] This implementation is currently used industry-wide by a number of manufacturers, provided by Monroe Shock Absorbers called CVSAe, or Continuously Variable Semi-Active electronic.

In , with the introduction of the Nissan GT-R, "DampTronic" was jointly developed by Nissan and Bilstein. DampTronic provides three selectable driver settings that can also interact with the Vehicle Dynamics Control technology to modify the transmission's shift points. The settings are labeled as Normal, Comfort, or R, and can be either set in Normal for automatic adjustment or the "R" setting for high-speed driving, while "Comfort" is for touring and a more compliant ride. The "R" mode enables the vehicle to utilize the yaw angle rate with a reduced steering angle for a crisper, more communicative steering, while the "Comfort" setting produces less vertical G-loading in comparison to the "Normal" or computer determined suspension setting.[25]

Magnetorheological damper

Another method incorporates magnetorheological dampers with a brand name MagneRide. It was initially developed by Delphi Corporation for GM and was standard, as many other new technologies, for Cadillac STS (from model ), and on some other GM models from . This was an upgrade for semi-active systems ("automatic road-sensing suspensions") used in upscale GM vehicles for decades. It allows, together with faster modern computers, changing the stiffness of all wheel suspensions independently. These dampers are finding increased usage in the US and already leases to some foreign brands, mostly in more expensive vehicles.[citation needed]

This system was in development for 25 years. The damper fluid contains metallic particles. Through the onboard computer, the dampers' compliance characteristics are controlled by an electromagnet. Essentially, increasing the current flow into the damper magnetic circuit increases the circuit magnetic flux. This in turn causes the metal particles to change their alignment, which increases fluid viscosity thereby raising the compression/rebound rates, while a decrease softens the effect of the dampers by aligning the particles in the opposite direction. If we imagine the metal particles as dinner plates then whilst aligned so they are on edge - viscosity is minimised. At the other end of the spectrum they will be aligned at 90 degrees so flat. Thus making the fluid much more viscous. It is the electric field produced by the electromagnet that changes the alignment of the metal particles. Information from wheel sensors (about suspension extension), steering, acceleration sensors - and other data, is used to calculate the optimal stiffness at that point in time. The fast reaction of the system (milliseconds) allows, for instance, making a softer passing by a single wheel over a bump in the road at a particular instant in time.[citation needed]

Production vehicles

By calendar year:

For more information, please visit adaptive suspension cars.

See also

References

How Lotus raised the bar on innovation with its active suspension &#; and made its mark 30 years ago at Monaco 

When did Formula 1 turn hi-tech rather than &#;merely&#; clever and cutting edge?

Ask Peter Wright. As the founding father of active suspension at Lotus, he should know.

&#;It began to get technical at the end of ,&#; he says. &#;We put a data system, which Cranfield Flight Instrumentation had built for us, on a car [Type 78] to measure and better understand ground effect.

&#;Out of that analysis came the definition of active.&#;

What followed was a gradual uncoiling: &#;At the start of , when we raced our first active car [Type 92-Cosworth], we had an onboard computer that wasn&#;t just recording them but having an effect on its dynamics.

&#;And it&#;s been hell ever since!&#;

Nigel Mansell finished 12th at Rio and Long Beach in this radical, spring-less machine &#; muscular hydraulics having replaced suspension&#;s &#;iron&#; skeleton &#; but remained far from convinced, spooked by the system&#;s inconsistencies.

Instead he had the green eye for team leader Elio de Angelis&#; Type 93T; though a dog, it had a Renault V6 turbo in its tail.

Turbo grunt was deemed more pressing in the late-notice flat-bottom era and active was summarily dropped &#; until General Motors, new owners of Group Lotus, and Ayrton Senna expressed an interest in -&#;87.

Wright was recalled to Team Lotus having spent the preceding four years honing active for road car use.

&#;[Laughs] Now I was trying to run an R&D department of 15, plus the F1 programme and the active system on the [IMSA] Corvette GTP car,&#; he says. &#;I didn&#;t have time to stop and think.

&#;I was the active suspension race engineer on Ayrton&#;s car. My job was to set up its suspension according to what he and [race engineer] Steve Hallam wanted.

&#;My side was doing data-analysis, too, and we fed that back to them. But they made the decisions and we executed them.

&#;In , we had started data-analysis because active has lots of sensors and, therefore, you get lots of info for nothing, as it were. We didn&#;t have telemetry; we had to download it to analyse it. We were using an analogue computer with a digital interface.

&#;We had a fully digitised car computer and data system by , unlike [new engine supplier] Honda who, on the other side of the garage, were plotting engine parameters on chart recorders. They photographed our data-analysis system like mad and about four races later had one just like it.

&#;The real potential of active was that you could change the attitude of the car to tune its aerodynamics. That&#;s why it got banned.

&#;You can vary front and rear ride heights according to speed, whether it was accelerating, braking or cornering. We didn&#;t have the simulation tools. We just had to learn, from scratch at every circuit, what worked where.

&#;We were all learning on the fly.

&#;[Technical Director] Gérard Ducarouge was very supportive. But I remember at a Donington Park test, at about midnight, when we were still trying to sort out the active and the turbo, &#;Duca&#; saying, &#;This used to be fun. It isn&#;t anymore.&#;&#;

Except when you&#;re winning.

Senna&#;s back-to-back victories in Type 99T on the streets of Monaco and Detroit were high points of an impressive learning year: 57 points, third in the championship.

&#;They were fantastic highs,&#; says Wright. &#;On his slow-down lap at Monaco, Ayrton was shouting over the radio, &#;Wow! Yeah! Man! This is better than sex!&#;

&#;It&#;s easy to see why motor racing hooks people in.

&#;Plus they were the only two times that I was at a race when Team Lotus won; I didn&#;t go racing in -&#;78.&#; (Wright was busy in Imperial College&#;s wind tunnel while the resultant Types 78 and 79 were blowing away the opposition.)

&#;Those wins were magical: Monaco, the most prestigious GP, and then Detroit, where all our engineering customers were &#; they meant a lot to Group Lotus.

&#;GM was already interested in active. They felt very threatened by the Japanese and were grasping for technology without, one might say, fully understanding what they were doing.&#;

It was to this &#;funny bunch&#; that Wright returned with Group Lotus when Team Lotus abandoned active for .

&#;That decision &#; a financial one, probably &#; was a mistake,&#; he says. &#;We had done all the hard work and were poised to make the most of it.

&#;Active suspension had already given us an advantage when tyres were marginal.&#;

Despite running non-stop in Detroit, Senna set fastest lap in response to Nigel Mansell&#;s charge immediately after pitting for fresh rubber for his pole-setting Williams FW11B-Honda.

The latter&#;s race was subsequently hampered by leg cramps &#; he limped home fifth &#; whereas Senna finished the race &#;feeling brand new&#;, his suspension having insulated him from Motown&#;s lumps and bumps.

At Monaco, he hadn&#;t even bothered to tape his gearchange hand.

&#;I don&#;t think a racing driver really minds about the ride,&#; says Wright. &#;He is only interested in grip. But the ride element of active affected how the tyres performed, albeit in ways that we did not yet fully understand.

&#;Ayrton did a very good analysis: &#;Ultimately, the car is as quick with active but, in qualifying, it takes much longer to get the tyres to work. Once you get there, however, you can sustain it, whereas the passive car hits a peak and falls off.&#;

&#;As such, he used to spend real effort on getting the race set-up right. He had a car that looked after its tyres and that&#;s what enabled him to win.

&#;But, being Monaco, the driver comes into it a hell of a lot, too.&#;

Senna&#;s move to McLaren for was a huge blow to Lotus.

&#;He said one or two things to me that indicated he didn&#;t think Team Lotus was doing the right thing,&#; says Wright. &#;Ayrton saw it coming.

&#;In some ways it was nice to leave [Team Lotus]. Though they were good days, they caused me a lot of white hair; had been particularly hair-raising because we were really experimenting.

&#;Had we been able to go on with active we would have understood what it was doing to tyres &#; and that would have been a big breakthrough.

&#;Team Lotus should have kept on with it &#; just when other teams were investing heavily &#; and gained an advantage. Maybe then it would have kept Senna and Honda.

&#;After active was banned in , Patrick Head told me that Williams would have had to use next a full active system like ours.&#;

By which time Mansell had completed F1&#;s then most dominant campaign thanks in the main to Williams&#; more pragmatic &#;reactive&#; suspension, and Wright was back with a Team Lotus in its death throes.

Though it had one more season left in its locker, the name that epitomised F1&#;s clever and cutting edge had scored its final GP victory: Senna at Detroit &#; three weeks after he had helped turn F1 properly hi-tech.

At Monaco, 30 years ago.

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